1. Field of the Invention
The present invention relates to a polytetrafluoroethylene (hereinafter referred to as "PTFE") porous film, a process for preparing the same and a filter comprising the same. More particularly, the present invention relates to a novel PTFE porous film useful as an air filter which is suitable for trapping suspended fine particles in air or other gases in an clean room to be used in a semiconductor industry and which causes a small pressure loss of the air or other gases.
2. Description of the Related Art
As a material of the air filter to be used in the clean room, a filter material prepared by forming a sheet from a mixture of glass fibers and a binder is often used. However, such filter material has some drawbacks, for example, presence of adhered minute fibers in the filter material, occurrence of self-dusting during processing or folding of the filter material, or increase of the pressure loss caused by an increased amount of the binder to be added to suppress the self-dusting (cf. Japanese Patent Kokai Publication No. 16019/1988 and corresponding U.S. Pat. No. 4,877,433). In addition, when such filter material is contacted with a certain chemical such as hydrofluoric acid, it makes dust due to deterioration of the glass and the binder.
To solve such drawbacks, an electret filter material made of a synthetic fiber is proposed in Japanese Patent Kokai Publication No. 53365/1979, but it suffers from deterioration of the electret.
To overcome the above defects, it is proposed to use a stretched porous film of PTFE as an auxiliary member for the filter material (cf. Japanese Patent Kokai Publication Nos. 16019/1988 and 284614/1990).
However, this proposal uses a porous PTFE film having a pore size of 1 .mu.m or larger to prevent increase of the pressure loss.
A theoretical reason for that the suspended particles having a particle size smaller than the above pore size can be trapped may be as follows:
There are following three mechanisms for removing the particles from a fluid by the filter (cf. a brochure of Domnick Hunter Filters Limited).